I play mostly guitar but sometimes bass, though I am a novice at it.
Lately, I've been putting in many hours on bass - it's fun and it would be very cool to be decent at both.

The thing is - these instruments play similar for the fingers, but the fret spacing is way larger on bass. When playing without looking, I will fret incorrectly all the time because my hands are used to guitar. My question basically is, thinking long term - will muscle memory for one instrument screw up the other? Is it best to focus exclusively on either instrument - any experiences with this?
If you want to play both, play both. As you gain experience and improve, you'll have less difficulty moving between instruments. And you'll likely find that playing bass will make you a better guitar player and vice versa.
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Quote by Tostitos
As you gain experience and improve, you'll have less difficulty moving between instruments.

Pretty much this.

A close friend of mine is a guitar virtuoso, and for many years he only played the guitar.
Recently he started playing bass in band, and now he's an excellent player on both instruments. He can switch between the two at any given moment and play either of them perfectly.

As with most technique-related questions, the answer is practice. Practice until your fingers fall off, and then keep practicing more.
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When I was getting used to bass after years of guitar I found it was much easier to play guitar first. That way my fingers are stretched and the guitar doesn’t feel tiny after playing bass. But you’ll get used to both after a year and be able to switch on the fly.
You must remember that while similar, they are two different instruments and take differnt skill sets to play. Some skills will obviously cross from one to the other but treat them like the individuals they are.
Maybe look into a short scale bass? They're super fun. I sold my full scale four string and wound up replacing it with a short scale, it's a lot of fun but I miss the P Bass I won't lie.
Started with guitar at 10...switched to bass at 15...the human brain is incredible...as soon as I pick up one it totally forgets about the other...